When adaptive antenna arrays are used, the basic principle is that radiation beams are narrow and they are directed as directly as possible at a desired receiver. Widely known methods of using adaptive antenna arrays can be divided into two main groups: radiation beams are directed at a receiver, or the most suitable beam is selected from various alternative beams. For a downlink transmission, a suitable beam is selected or the beam is turned on the basis of information obtained from the uplink. The reuse of frequencies can be made more efficient and the power of transmitters can be reduced, because, owing to the directivity of antenna beams, interference with other users diminishes.
In a digital system, the directing of antenna beams in the up-link is implemented by dividing a signal in baseband parts into I and Q branches and by multiplying in a complex manner (phase and amplitude) the signal of each antenna element by appropriate weighting coefficients, and then by summing up the output signals of all antenna elements. In this case, an adaptive antenna array comprises not only antennas but also a signal processor, which automatically adapts antenna beams by using a control algorithm so that in the downlink, it turns the antenna beams to a direction in which the strongest signal is measured in the uplink. The directivity of beams can also be implemented analogically by using fixed phasing circuits (Butler matrix) for generating orthogonal radiation beams in which the phase changes linearly antenna by antenna. The Bulter matrix is used to measure which beam receives most signal energy, i.e. in which beam the signal is the strongest, and this beam is selected for the transmission.
However, it is not always sufficient that a user-specific antenna beam is used for monitoring the location of the user as precisely as possible. Namely, the problem is that the number of those needing radio resources varies in the coverage area. A radio cell, where the number of those needing services can usually be covered, can temporarily become a so-called hot spot area with a lot of users in a relatively small area. To solve this problem, it has been suggested that an antenna beam, i.e. sector, formed with a fixed Bulter matrix is split into two beams. The method allows the operator to change azimuth angles by a step of 30 degrees, since 4 orthogonal beams, each having a coverage area of about 30 degrees, can typically be generated with a 4×4 Butler matrix. The method is called sector synthesis.
A problem with fixed-beam systems is, however, how to design power amplifiers, because, in the worst case, each power amplifier of an antenna array has different designs, which causes costs during planning and implementation phases. In digital beam forming, the problem with an attempt to have a power balance between different antenna elements is that compromises have to be made in order to optimize the width and side-lobe levels of antenna beams.